[docs]: document default behaviors

# Documentation request

Cloud-init does several things on first boot without any configuration that a user may want to be aware of.

Some of these behaviors are cloud-specific (ex: azure, cloudstack: setting default password).
Some of these behaviors occur on most or all clouds (ex: generate ssh host keys, install user-provided ssh public keys, etc).

Many default behaviors may be modified or overridden using cloud-config.

The problem is that most users are not aware of what cloud-init does by default, and this is not documented. A new explanation page that describes cloud-init's default behaviors would be very useful.

A good start to documenting this would be to create a short summary of what happens when no configuration is passed in each cloud-config module whose metaschema contains `"activate_by_schema_keys": [],`.